Investigations for safety assessment of welded components made of 15 NiCuMoNb 5 (WB 36) with respect to operation-induced loss in toughness

The welded products used for the investigations reported, relating to operation-induced changes of materials properties at operating temperatures exceeding 300 C, are a sheet and two seamless pipes, each not used, and two burn-out specimens taken from one out of four boiler cylinders with circumferential and longitudinal welded seams dismantled after 57,000 operating hours. Except for the sheet which meets practically all quality requirements of the KTA safety code, the quality of the products examined corresponds to that of older LWR plants. In the two burner cylinder specimens, an operating temperature of 350 C induced a varying increase in the strength of materials, accompanied by a decrease in the impact energy, primarily in the transition zone. Such changes could be observed also in the three non-used welded specimens, after long-term annealing treatments from 18,000 h at 335 C up to a maximum of 3,000 h at 400 C. Whereas the operation-induced increase in materials strength in most cases was within the values known to be normal, the impact energy values at 0 C were below the value of ≥31 J required in the VdTUeV-WB.377 publication. This effect was sometimes found also in the optimized welded specimens, for which the impact energy should be ≥41 J according to KTA 3211.1, although their impact energy values were distinctly higher than in the conventional specimens. The operation-induced changes in the materials' toughness (KIJ values) were even more serious than those in the impact energy, both in the transition zone as well as at Charpy upper-shelf energies, with the decrease being particularly clear at 350 C